Prompt Engineering

Explain API Flow — One Endpoint, End to End

What actually happens when a client calls POST /orders: the contract, every decision point, every side effect, and the conditions that route off the happy path.

Overview

Endpoint behavior lives in many places — validation, services, queues — and explanations that stay inside one function miss the flow. This prompt uses the function-walkthrough mode on an end-to-end objective: the contract first (inputs with shapes, outputs with meaning, errors with triggers), the execution flow walked in order with both branches explained at every decision point, and every side effect surfaced — writes, calls to other systems, queued work — with when each happens. The happy path gets distinguished from the edge paths by the conditions that separate them. The result reads like the sequence diagram nobody drew.

Workflow

Contract first
Inputs with expected shapes, outputs with meaning, errors with their triggers — before any flow.
Walk every decision
Each branch explained in plain language: the condition, and what each outcome means.
Surface the side effects
Writes, external calls, queued jobs — and exactly when in the flow each one happens.

Why This Works

Contract-first framing matches how API consumers actually think
Both-branches discipline catches the error paths narration skips
Side-effect timing is where endpoint surprises actually live

Best for

Endpoints whose behavior spans services and queues
Pre-change understanding of critical API paths
Debugging-adjacent understanding without an active failure

Not for

Investigating an endpoint that is failing right now — that's the Debugging Prompt Generator
Documenting the API for consumers — that's the Markdown Output Builder's API documentation space

Use cases

Understanding what POST /orders really does before changing it
Surfacing the side effects an endpoint hides
Mapping the error responses to their actual triggers

Customize This Resource

Opens this setup in Code Explanation Prompt. Generate to get the full understanding contract — then adjust the mode, audience, and depth.

Open in Code Explanation Prompt

Prompt Template

EXPLANATION OBJECTIVE
Explain what happens end to end when a client calls POST /orders.
Explanation mode: function walkthrough — inputs, outputs, execution flow, decisions, and side effects.
The goal is understanding, not judgment: explain what this code does and why it is built this way — and clearly separate what the code shows from what you infer.

CODE CONTEXT
System context: REST API; the handler validates, reserves inventory, charges payment, and queues fulfillment.
Code to explain:
```
[Paste the code to explain here]
```

AUDIENCE PROFILE
Audience: a mid-level developer — fluent in the language and common patterns.
- Assume: language features and common patterns are known; this codebase's conventions and domain are not.
- Skip syntax explanations entirely; spend the depth on intent, structure, and domain meaning.

EXPLANATION STRATEGY
1. State the contract first: inputs with their expected shapes, outputs with their meaning, errors with their triggers.
2. Walk the execution flow in order, pausing at every decision point to explain both branches.
3. Surface every side effect — writes, calls to other systems, mutations of shared state — and when each happens.
4. Distinguish the happy path from the edge paths; name the conditions that route execution off the happy path.
- Depth: detailed explanation — thorough coverage of behavior and structure.
- Cover every section of this contract with real content — no skipped or token sections.
- Go as deep as the code requires, but no deeper: detail serves understanding, not completeness theater.

KEY CONCEPTS
- Explain each non-obvious variable, structure, and helper the walkthrough touches — what it holds and why it exists.
- Order concepts by how load-bearing they are for understanding this code — not by where they appear in the file.

EXECUTION FLOW
- Present the flow as a numbered sequence; at each branch, state the condition in plain language and what each outcome means.
- Where the flow depends on state or configuration, say which, and what each setting changes.

DESIGN DECISIONS
- Where the implementation makes a visible choice — an early return, a special case, a particular ordering — explain what the choice achieves.
- Discuss only decisions actually visible in the code; rationale you cannot evidence belongs under ASSUMPTIONS, labeled as inference.

ASSUMPTIONS
- Separate what the code DOES (verifiable from the source) from why it might be that way (inference).
- Label every statement about intent, history, or rationale as an INFERENCE unless the code itself evidences it — a comment, a test, a telling name.
- Where behavior cannot be determined from the provided code, say "cannot be determined from this code" — never fill the gap with a plausible story.

TRADEOFFS
- Identify the tradeoffs this code embodies: what it optimizes for, and what it sacrifices for that.
- Tie each tradeoff to its visible consequence in the code.

LEARNING NOTES
- End with the contract restated in two sentences: what you give it, what you get back, and what else changes.

NON-GOALS
- Do not review code quality — no findings, no severities, no verdicts.
- Do not debug failures — explain the behavior that exists without diagnosing it.
- Do not refactor the code or propose rewrites.
- Do not generate tests.
- Do not invent requirements or unstated intentions.
- If something looks defective, note it as a question for the team — investigating or fixing it is out of scope.
- Clearly separate facts from assumptions throughout.

OUTPUT REQUIREMENTS
- Structure the explanation with headings that mirror the sections above — a reader should be able to navigate it without reading all of it.
- Anchor every claim about behavior in the code: name the function, branch, or line it comes from when precision matters.
- Calibrate every section to the stated audience — too basic wastes their time, too advanced loses them.
- End with the open questions: what could not be determined from the provided code, and who or what could answer it.

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